1. Field of the Invention
The invention relates to a device for producing crystal rods of a defined cross-section and columnar polycrystalline structure by floating-zone continuous crystallization (CCC=crucible-fed continuous crystallization), provided with at least a crucible filled with crystal material and provided with a central output for feeding the content of the crucible to a growing crystal rod positioned under the crucible, the central output being immersed in the melt meniscus on the upper surface of the crystal rod, means for the continuous controllable feeding of the crucible with solid crystal material and means for the simultaneous feeding of the melt energy and setting of the crystallization front.
2. The Prior Art
In accordance with the prior art, the crystallization of polycrystalline block silicon from silicon granulate takes place in a temporal sequence of melting the raw material and the subsequent crystallization of the melt or by simultaneous melting and crystallizing in a direct thermal interaction.
Thus, 11th E.C. Photovoltaic Solar Energy Conference, 12-16 Oct. 1992, Montreux, Switzerland, pp. 1070-1073, describes an arrangement for producing polycrystalline block silicon by electromagnetic continuous casting (EMC—Electromagnetic Casting). The melt is enclosed by a tubular round or square induction coil (the principle of the cold reflector) provided for melting of the fed pieces of silicon and, at the same time, for the crystallization of the crystal rod which is continuously pulled from below in a downward direction. The high frequency field of the induction coil generates electromagnetic forces which keep the melt away from the wall of the crucible and which form the block silicon. The heater enclosing the silicon crystal rod and the support of the melt by magnetic forces are characteristic of this arrangement.
However, owing to the lateral heat infusion the phase boundary is strongly parabolically bent which leads to extreme thermo-mechanical stresses in the polycrystalline rod. Also, the majority of the crystals is not axially (columnar) oriented which reduces the effectiveness of solar cells made of this material. This effect increases with increasing drawing speed and larger rod cross-sections. Hence, for technically relevant cross-sections the drawing speed is limited to 0.8 to 1.2 mm/min.
In the FZ-method described in DE 195 38 020A1 the melt is heated by a resistance heater and the required energy infusion into the crystallization front is carried out by an induction heating coil, which is preferably structured as a plate-shaped flat coil with a central inner opening. This technically complex heating arrangement consisting of two separate heating means serves the controlled post-charging of silicon granulate as well as the prevention of undercooling of the crystal rod in the production of rod-shaped silicon mono-crystals of large diameters.
The proceedings of the 16th European Photovoltaic Solar Energy Conference, 1 to 5 May 200, Glasgow, UK, pp. 1616-1619, describe a system for producing crystal rods by inductive top-heated continuous crystallization (ITCC). The arrangement is provided with a funnel to which silicon granulate is fed and the tube of which terminates closely above the surface of the melt at the upper end of the crystal rod. Solid raw material pre-heated by a heat lamp is fed to the funnel by a feed conduit. The material drops through the funnel, which is surrounded by a flat annular inductor, onto the melt on the silicon crystal rod where the inductive heat causes the material flowing there to be melted. 1 to 2 cm below the liquid/solid phase the cylindrical polycrystalline silicon rod crystallizes while rotating about its axis. This arrangement makes possible a lesser bent phase boundary and crystallites of corresponding columnar orientation at a growth rate of at most 1 to 1.5 mm/min because the heating energy for melting a large quantity of raw material at the same time lowers the undercooling of the melt and, therefore, the rate of crystallization. However, this arrangement only allows the fabrication of crystal rods of circular cross-section.
The status report 1996 Potovoltaik presented by the project sponsor Biology, Energy, Ecology of the (German) Federal Ministry of Education, Science, Research and Technology—Research Center Juelich GmbH, on the occasion of the status seminar Photovoltaik 1996 at Bad Breisig (Germany) from 23 to 25 Apr. 1996, 5-1 to 5-11, refers to a crucible-fed continuous crystallization of silicon as well. It discloses an arrangement with a surface lamp heater for the crucible-fed continuous crystallization of silicon rods of round cross-section. The heat energy of the lamps is directed towards the upper end surface of the silicon rod; further lamps are disposed at the periphery and serve to heat the melt and the post-heating of the rod following re-crystallization. A frame of non-melting material is arranged above the silicon rod; it is partially submerged in the melt and acts as a shape-imparting element. By this arrangement, crystal rods of square cross-section may also be produced. The setting of a defined heating power poses problems, however, since at too high a heating power, the height of the free melt between the silicon rod and the frame increases, and liquid silicon may escape. At too low a heating power, the silicon rod may grow into the frame. To avoid the disadvantages of feeding cold granulate, a flat tub of graphite was arranged above the frame for dispensing, by way of a central bore, the melt provided for a liquid after-charge, into the shape-imparting frame. As may be seen, the setting of a defined/optimal heating power is difficult and was here realized by complex means, i.e. focused lamp radiation and an additional reflector. As has already been mentioned supra, two optical heaters were applied, one being used for melting the raw material, the other one for controlling the crystallization of the silicon rod to be produced.
Another possibility for a crucible-fed continuous crystallization of silicon is described in the publication mentioned above and relates to a float-zone crystal drawing arrangement. Above an induction coil, a storage bunker is provided within a receptacle for receiving the silicon granulate for the after-charging. The granulate was fed by way of a quartz funnel through openings in the inductor to the surface of the silicon rod where it is almost completely melted. Several silicon grains did, however, escape to the edge of the rod where they nucleated. In order to achieve complete melting of the granulate, it was fed through a quartz tube protruding through the inductor and touching the melt on the silicon rod. While this results in a good columnar grain structure, melting of the material as well as crystallization in this arrangement take place below the frame. The energy distribution of the heating arrangement limits the melting rate as well as rate of crystallization. No shape-imparting frame was used in respect of the second-mentioned arrangement.